From: Peter Zijlstra peterz@infradead.org
commit e52c1dc7455d32c8a55f9949d300e5e87d011fa6 upstream.
FineIBT-paranoid was using the retpoline bytes for the paranoid check, disabling retpolines, because all parts that have IBT also have eIBRS and thus don't need no stinking retpolines.
Except... ITS needs the retpolines for indirect calls must not be in the first half of a cacheline :-/
So what was the paranoid call sequence:
<fineibt_paranoid_start>: 0: 41 ba 78 56 34 12 mov $0x12345678, %r10d 6: 45 3b 53 f7 cmp -0x9(%r11), %r10d a: 4d 8d 5b <f0> lea -0x10(%r11), %r11 e: 75 fd jne d <fineibt_paranoid_start+0xd> 10: 41 ff d3 call *%r11 13: 90 nop
Now becomes:
<fineibt_paranoid_start>: 0: 41 ba 78 56 34 12 mov $0x12345678, %r10d 6: 45 3b 53 f7 cmp -0x9(%r11), %r10d a: 4d 8d 5b f0 lea -0x10(%r11), %r11 e: 2e e8 XX XX XX XX cs call __x86_indirect_paranoid_thunk_r11
Where the paranoid_thunk looks like:
1d: <ea> (bad) __x86_indirect_paranoid_thunk_r11: 1e: 75 fd jne 1d __x86_indirect_its_thunk_r11: 20: 41 ff eb jmp *%r11 23: cc int3
[ dhansen: remove initialization to false ]
[ pawan: move the its_static_thunk() definition to alternative.c. This is done to avoid a build failure due to circular dependency between kernel.h(asm-generic/bug.h) and asm/alternative.h which is needed for WARN_ONCE(). ]
[ Just a portion of the original commit, in order to fix a build issue in stable kernels due to backports ]
Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Reviewed-by: Alexandre Chartre alexandre.chartre@oracle.com Tested-by: Holger Hoffstätte holger@applied-asynchrony.com Link: https://lore.kernel.org/r/20250514113952.GB16434@noisy.programming.kicks-ass... Signed-off-by: Pawan Gupta pawan.kumar.gupta@linux.intel.com --- arch/x86/include/asm/alternative.h | 2 ++ arch/x86/kernel/alternative.c | 19 ++++++++++++++++++- arch/x86/net/bpf_jit_comp.c | 2 +- 3 files changed, 21 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h index 4ab021fd3887..f2cce0cd87a6 100644 --- a/arch/x86/include/asm/alternative.h +++ b/arch/x86/include/asm/alternative.h @@ -80,6 +80,8 @@ extern void apply_returns(s32 *start, s32 *end);
struct module;
+extern u8 *its_static_thunk(int reg); + #ifdef CONFIG_MITIGATION_ITS extern void its_init_mod(struct module *mod); extern void its_fini_mod(struct module *mod); diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 923b4d1e98db..30dc73210c2e 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -752,7 +752,24 @@ static bool cpu_wants_indirect_its_thunk_at(unsigned long addr, int reg) /* Lower-half of the cacheline? */ return !(addr & 0x20); } -#endif + +u8 *its_static_thunk(int reg) +{ + u8 *thunk = __x86_indirect_its_thunk_array[reg]; + + return thunk; +} + +#else /* CONFIG_MITIGATION_ITS */ + +u8 *its_static_thunk(int reg) +{ + WARN_ONCE(1, "ITS not compiled in"); + + return NULL; +} + +#endif /* CONFIG_MITIGATION_ITS */
/* * Rewrite the compiler generated retpoline thunk calls. diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index c32270212640..d3450088569f 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -390,7 +390,7 @@ static void emit_indirect_jump(u8 **pprog, int reg, u8 *ip) if (IS_ENABLED(CONFIG_MITIGATION_ITS) && cpu_feature_enabled(X86_FEATURE_INDIRECT_THUNK_ITS)) { OPTIMIZER_HIDE_VAR(reg); - emit_jump(&prog, &__x86_indirect_its_thunk_array[reg], ip); + emit_jump(&prog, its_static_thunk(reg), ip); } else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_LFENCE)) { EMIT_LFENCE(); EMIT2(0xFF, 0xE0 + reg);